Section 69. The members of the board shall serve without compensation, except that there shall be paid by the commonwealth to the secretary, whether a member of the board or not, a salary of five hundred dollars. Each member shall receive from the commonwealth the necessary travel and other expenses actually incurred by him in the performance of his duties. The board may appoint such clerks as may be necessary; provided that the salary of the secretary and expenses of the members of the board and its employees, and the expenses of the board, shall not be in excess of the receipts for registration and from other sources that have been received by the state treasurer from the board.